Allergy Overview

What is an Allergy?

In basic language, an allergy results from the body's immune system recognizing a foreign molecule (termed an “antigen”) and creating an immune response against that antigen. When antigens are from infectious or otherwise harmful agents, such a response is beneficial; however, when the agent is otherwise harmless (for example, peanut proteins), the reaction is classified as an allergy. The immune system is massively complex, and immune responses can vary depending on the initial allergic reaction, the frequency of repeated reactions, and even host genetic factors. In general, immune reactions may involve the production of antibodies, immune complex formation, the activation of immune cells (T cells), or a combination of these mechanisms.

Drug Allergies

Drug allergies fall within the general category of adverse drug reactions (ADRs). In clinical practice, it is crucial to identify drugs that may cause an allergic reaction to prevent harm to the patient. In patients who have previously experienced an allergic drug reaction, it is also vital to determine drugs that may cause a similar allergic reaction (cross-sensitive drugs), which the patient should avoid.

Rapidly accessible information regarding allergic reactions following the administration of a particular drug is often limited to drug prescribing information. DrugBank has reviewed the literature and summarized available allergy information from prescribing information, journal articles, and other reputable sources. Extensive literature searches have been performed for drugs that commonly cause hypersensitivity (allergic) reactions. Our allergy dataset provides a centralized resource for allergic reactions, clinical presentations, and cross-sensitivities related to specific drugs.

Allergy Classification

Drug allergies can generally be classified into four types (type I-IV), depending on their underlying immunological mechanism. This classification system, known as the Gell and Coombs system, defines these types as shown below. We also define a fifth type within DrugBank: “Unclassified.”

Hypersensitivity typeOnset timeDescription
Type I<1 hourType I reactions, sometimes referred to as “immediate hypersensitivity reactions,” occur rapidly following drug administration. At the molecular level, type I reactions are mediated when a drug-related antigen cross-links IgE antibodies on the surface of mast cells, inducing the release of pro-inflammatory molecules.
Type IIHours to daysType II reactions occur when drug-induced antibodies (usually IgG or IgM) bind to cellular or extracellular targets causing cellular damage. This damage can occur due to direct binding effects, activation of the complement pathway, or antibody-dependent cytotoxicity.
Type III1-3 weeksType III reactions are mediated by the formation of antigen-antibody aggregates, termed immune complexes, which are inadequately cleared and accumulate in tissues. Immune complex deposition leads to complement pathway activation and downstream tissue damage.
Type IVDays to weeksType IV reactions, also known as “delayed hypersensitivity reactions,” involve the activation of T cells by a drug-related antigen. T cell activation leads to myriad downstream effects resulting in a pro-inflammatory state and tissue damage. Depending on the exact cell types involved, type IV reactions may be further classified into types IVa-IVd, though we consider only the general type IV.
UnclassifiedVariableIn most cases, this signifies that the hypersensitivity reaction type is not clearly identified in the literature. In other cases, the clinical presentation may not fall under any of the four hypersensitivity types (e.g. a clinically relevant non-allergic drug hypersensitivity reaction).

Allergy Categories

Allergy categories represent groups of drugs with similar characteristics such as common allergic reactions, clinical presentations, and cross-sensitivities. Although these allergy categories are not explicitly shown through our API, it is important to note that they are distinct from drug categories within DrugBank. Our allergy categories have been carefully chosen and reviewed by the curation team at DrugBank to ensure that the information can be accurately applied to all drugs within the category.

Drugs within an allergy category will often have common chemical/structural elements. One example of an allergy category is the category “Sulfonamide (antibiotic),” which groups together sulfonamide drugs used as antibiotics. These drugs have similar hypersensitivity and cross-sensitivity reactions. For example, due to similarities in chemical structure, sulfonamide antibiotics may show cross-sensitivity with amprenavir, an HIV drug.


Allergy information in DrugBank, including allergy details, presentations, and cross-sensitivities, is associated with in-text reference identifiers. These identifiers can then be found in the references section for further information about the literature reviewed. By default, all API calls within the allergy module have references turned off. To turn on the references, please see here: API support